Inertial platform trimming system



April 16, 1968' SHU LEE 1 INERTIAL PLATFORM TRIMMING SYSTEM OriginalFiled Jan. 5, 1961 5H0 55 MLM HTTOEA/EY Y'INVENTOR.

United States Patent 22 Claims. (Cl. 73-178) My invention relates to aninertial platform trimming system and more particularly to an automatictrimming system for an inertial platform having singlc-degree-offreedomgyroscopes which substantially eliminates gyroscope drift. Thisapplication is a continuation of my copending application for InertialPlatform Trimming System filed Jan. 5, 1961, Ser. No. 80,805, nowabandoned.

In the prior art gyro drift has been calculated or experimentallydetermined and signals to correct for such drift have been introducedinto the gyroscope. However, such methods of compensating for gyro driftdo not take into consideration the fact that the drift rate of gyroschanges from day to day and a value of calculated or experimentallydetermined gyro drift for one day will not apply for succeeding days.

I have invented an automatic trimming system for an inertial platform inwhich trimming is accomplished While the gyros are mounted on theplatform. N 0 external or auxiliary equipment is required and the gyrosneed not be removed from the platform.

One object of my invention is to provide an inertial platform trimmingsystem which automatically compensates for and eliminates gyro drift.

Another object of my invention is to provide a selftrimmed inertialplatform which does not require the use of auxiliary external equipment.

A further object of my invention is to provide an inertial platformtrimming system in which the gyros remain fixed on the platform and neednot be removed for the purpose of trimming.

Other and further objects of my invention will appear from the followingdescription.

In general my invention contemplates the provision of a conventionalinertial platform provided with a first and a second gyroscope. Thefirst and secon'd gyroscope's have orthogonally disposed input axeslying in a nominally horizontal plane. The platform is provided with afirst accelerometer having a sensitive axis parallel to the input axisof the second gyroscope and a second accelerometer having a sensitiveaxis parallel to the input axis of the first gyroscope. The firstaccelerometer is connected to torque the first gyroscope; and the secondaccelerometer is connected to torque the second gyroscope. The platformis further provided with an azimuth gyroscope having a vertical inputaxis which is nominally aligned with the earths gravitational vector.Into the gyroscopes are introduced appropriate torquing signals whichrotate the platform in accordance with the earths rate of rotation. Inthe absence of gyro drift the accelerometers cause the platform tomaintain a local horizontal attitude. If the sensitive axis of the firstaccelerometer and the input axis of the second gyroscope are alignedwith North, no earth rate is sensed by the first gyroscope. I couple theoutput of the first accelerometer to the azimuth gyro which in turnforces the platform to maintain, as a gyr'o co'mpass, the Northerlyorientation Olf the sensitive axis of the first accelerometer. No earthrate torquing signal need by introduced into the first gyroscope. Iintegrate the output of the first accelerometer and apply suchintegrated signal to the azimuth gyro. I integrate the 3,377,854Patented Apr. 16, 1968 'ice output of the second accelerometer andcouple such integrated signal to the second gyroscope. With theconfiguration thus far described, the plat-form, that is the planedetermined by the axes of the gyroscope and accelerometers, will assumea local horizontal attitude; but the axes of the first accelerometer andsecond gyrosc'olpe will deviate from true North proportionally to thedrift in the first gyroscope which controls gyro-compassing. Theintegrated signal applied to the azimuth gyro will, however, entirelycompensate for drift in such gyro. The integrated signal coupled to thesecond gyro will eliminate the major portion of drift in such gyro.However, a residual drift error remains which varies as the square ofthe error in the Northerly orientation of the sensitive axis of thefirst accelerometer. Generally the residual drift in the second gyrowill be extremely small compared with its normal drift. Accordingly, asa first approximation it may be assumed that the second gyro isdrift-free. I now cause the second accelerometer to control the azimuthgyro and disable the integrator associated with the second gyro tomaintain the existing drift correction signal. I now integrate theoutput of the first accelerometer and couple such integrated signal tothe first gyro. I switch the earth rate torquing signal from the secondgyro to the first gyro. The platform will now rotate in a horizontalplate through about the vertical input axis of the azimuth gyro so thatthe sensitive axis of the second accelerometer, which is parallel to theinput axis of the first gyroscope, is substantially aligned with North.Since the second gyroscope is substantially free of drift, the error inalignment of the sensitive axis of the second accelerometer with NorthWill be very small; and the first gyroscope will be compensated almostcompletely for drift, since its residual drift varies as a square of theextremely small angular error from North in the sensitive axis of thesecond accelerometer. I may then restore all connections to theiroriginal condition, whereupon the platform returns to its initialorientation with the sen'sitive axis of the first accelerometer now insubstantially perfect alignment with North, since the first gyroscope isnow compensated almost completely for drift. By such alternate switchingback and forth between the two conditions, the residual drift in thefirst and second gyros converges very rapidly upon zero as the number ofswitching operations is increased.

The accompanying drawing which forms part of the instant specificationandis to be read in conjunction therewith is a schematic view showingone embodiment of my invention.

Referring now to the drawing, I provide a conventional stabilizedplatform well known to the art as shown, for example, in the SutherlandPatent 2,955,474 upon which are mounted a gyro 21 having a horizontalinput axi a gyro 51 having an orthogonally disposed horizontal inputaxis, a linear accelerometer 24 having a sensitive axis parallel to theinput axis of gyro 51, a linear accelerometer 54 having a sensitive axisparallel to the input axis of gyro 21, and an azimuth gyro 81 having avertical input axis aligned with that of the earths gravitationalvector. Gyros 21, 51, and 81 are singledegree-of-freedom gyroscopeswhich provide an output signal whenever the gimbal deviates from itsnull position. The gyroscopes are further provided with input windingswhich, as well known to the art, produce torques about the gimbal oroutput axis. The output signal of the gyro 21 is coupled to an amplifier22 which drives a servomotor 23. Servo 23 controls the attitude of theplatform about the input axis of gyro 21. Accelerometer 24 is alsoresponsive to the component of gravity which accompanics a platform tiltabout the input axis of gyro 21. The output of accelerometer 24 iscoupled through an input resistor 25 to the input terminal of ahigh-gain feedback amplifier 26. The output of amplifier as is coupledthrough a feedback resistor 27 to its input terminal. The output ofamplifier 26 is also coupled to one torquing input of horizontal gyro21. The closed loop 2t? includes a horizontal gyro 2i and accelerometer2d. The output signal of horizontal gyro fill is coupled to an amplifier52 which drives a servomotor 53. Servomotor 53 controls the attitude ofthe platform about the input axis of gyro SE, to which tiltsaccelerometer 54 is responsive by virtue of the resulting gravitycomponent. The output of accelerometer $4 is coupled through an inputresistor 55 to the input of a high-gain feedback amplifier 56, theoutput of which is coupled to its input through a feedback resistor 57.The output of amplifier 56 is coupled to one torquing input ofhorizontal gyro 51. The closed loop 5d includes accelerometer 54 andhorizontal gyro ST. The mechanical arrangement forms no part of myinvention. 1 may employ various configurations well-known to the artsuch as one shown in the Sutherland Patent 2,955,474. I provide anavigation co-. puter indicated generally by the reference numeral 13,which includes a resolver 15. I provide an electrical input 11 toresolver which is precisely scaled to represent the earth rate w. Iprovide a mechanical input 12 to resolver 15 which represents the knownlatitude L of the platform on the surface of the earth. In theparticular embodiment of my invention shown, the platform is at restrelative to the earth and has no translational velocity. Resolver 15,having inputs w and L, computes the quantity 16 representing the productw sin L, and the quantity 17 representing the product w cos L. Thequantity 16 is coupled to one torquing input of azimuth gyro 81. Thequantity 17 is coupled to the armature of a double-pole, single-throwswitch indicated generally by the reference numeral it The upper contactof switch 10 is connected to a second torquing input of horizontal gyro51. The lower contact of switch 10 is connected to a second torquinginput of horizontal gyro 21. The output of amplifier is coupled to theupper contact of a single-pole, double-throw switch indicated generallyby the reference numeral 90. The armature of switch 90 is connected to asecond torquing input of azimuth gyro fill. The lower contact of switch9%) is connected to the output of amplifier 56. The signal at thearmature, switch 90, is coupled to an amplifier 1 which drives avelocity servo 92. The output shaft of velocity servo 92 rotates at aspeed proportional to the signal existing at the armature of switch 99.The output shaft of velocity servo 92 drives a brush 93 of apotentiometer 94. Brush 93 is connected to a third input of azimuth gyro31. The output of amplifier 26 is connected to the armature of a switchindicated generally by the reference numeral 39. The lower contact ofswitch 3% is connected to an amplifier 31 which drives a velocity servo32. The output shaft of velocity servo 32 moves with an angular speedproportional to the input signal of amplifier 31. The output shaft ofvelocity servo 32 drives a brush 33 associated with potentiometer 34.The brush 33 is connected to a third input of horizontal gyro 21. Theinput of amplifier 31 may be connected by means of a manually operablenormally open switch, indicated generally by the reference numeral 38 tobrush 33. The out put of amplifier 56 is connected to the armature of aswitch indicated generally by the reference numeral so. The uppercontact of switch 60 is connected to an amplifier 61 which drives avelocity servo 62. The output shaft of velocity servo 62 moves with aspeed proportional to the signal existing at the input of amplifier 61.The output shaft of velocity servo 62 drives a brush 63 associated witha potentiometer 6 5. Brush 63 is connected to a third input ofhorizontal gyro 51. I provide batteries 1 and 2 for generating a stabledrift-free reference voltage. The negative terminal of battery 1 and thepositive terminal of battery 2 are grounded. The positive terminal ofbattery 1 is connected to one terminal of each of potentiometers 34, 6and M2. The negative terminal of battery 2 is connected to the otherterminal of each of potentiometers 34, 64, and 94. Each ofpotentiometers 34, 64-, and W. is provided with a center tap which isconnected to ground. Switches it), 39, 6t), and are all controlled by arelay actuating winding 9. The upper contact of switch 30 and the lowercontact of switch 66 have no connections. I provide a constantspeedtiming motor 5 which is supplied by a battery 5 through a manuallyoperable switch 4. Battery 3 also excites relay winding 9 in series witha resistor 13 through a normally closed micro-switch 8 having anactuating arm 7. Timing motor 5 drives a cam 6 upon which ridesmicro-switch actuating arm 7. In the position of cam 6, as shown,micro-switch 8 is open circuited. Relay actuating winding 9 is notenergized, which causes switches iii, 3%, 60, and 96 to be in theirupper positions, as shown.

in the upper position of switches it 3%, 6i and 90, as shown, loop 2%including gyro 2i and accelerometer 24, controls azimuth gyro-compass81. Accordingly the platform rotates azimuthally in seeking a null ofthe earth rate component about the input axis of gyro 21 so that loop 23will be responsive to north-south platform tilts, while loop 5%including horizontal gyro 5?. and accelerometer 54, will be responsiveto cast-west platform tilts. An earth rate signal is introduced onlyinto loop 5i and not into loop 2d.

In operation of my inertial platform trimming system, switch 38 isclosed, causing the signal at brush 33 to drive the velocity servo 32until the signal is nulled and the output at brush 33 is zero. Switch 33is then opened. Since the drift of gyro 21 is unknown, it would beundesirable that any drift compensating signal be present at brush 33.Switch 4 is then closed, energizing timing motor 5, which drives cam 6at a constant speed. The signals from amplifier 56 and from brush 63introduced into gyro 51 cause the platform to stabilize with noeast-west tilt by virtue of the earth rotation compensating signal fromresolver 1S which is also coupled to gyro 51. Tie output signal ofamplifier 26 applied through switch 99 to azimuth gyro-compass 81 andthe integratedsignal at brush 93 causes the platform to orient itselfvery close to true north. The error from true north is that necessary tosubject horizontal gyro 21 to an earth rate influence sulficient tocompletely cancel the drift in gyro 23, whereupon loop Ztl will have nonorth-south tilt. The entire drift correction for azimuth gyro-compass31 will be supplied at brush 533, since the outputs of accelerometer 2dand amplifier 26 are zero. The major portion of drift compensation forgyro 51 will be supplied from brush 65. However, a second-orderresidual, uncompensated drift will exist in horizontal gyro 51 due tothe error in northerly orientation of the platform necessary to cancelthe drift in gyro 21. During the period in which the platform settlesdown to a fixed orientation which is only slightly in error from truenorth and in which the platform settles down to a completely levelcondition with no tilt, timing motor 5 rotates cam 6 counterclockwise.Micro-switch 8 now closes, energizing winding 9 and drawing thearniatures of switches iii, 3h, as, 90 into engagement with the lowercontacts. This immediately opens switch 6t) thereby disabling velocityservo 62 and maintaining at brush 63 the voltage which almost exactlycancels the drift in gyro 51. The earth rate correction is disconnectedfrom gyro Si and connected to gyro 21. The actuation of switch 99 causesloop Silto control azimuth gyro-compass 81. The closing of switch 39enables velocity servo 32 to provide drift corrections to gyro 21. Theplatform turns gradually through 90, so that loop 50 is responsive tonorth-south tilts and loop 20 is responsive to east-west tilts. Loop St)has very little residual gyro drift. Accordingly, the northerlyorientation of the platform is now substantially better than before,when the switches were in their upper position, since gyro 51 issubjected only to that earth rate influence necessary to cancel itsresidual second-order drift. The platform stabilizes with no tilt andwith the signal at brush 33 substantially eliminating drift in gyro 21.The trimming operation may now be stopped by opening switch 4, if thelatitude of the earth-slaved platform is appreciably less than 90.However, at very high latitudes, it may be desirable to perform anadditional half-cycle of operation. If switch 4 is left closed, thentiming motor 5 will continue to rotate cam 6, opening micro-switch 8 andde-energizing winding 9. The armatures of switches 10, 30, 60, and 90are returned to their upper position, as shown, where loop 20 controlsgyro-compassing. The platform now turns gradually back 90 to itsoriginal orientation where loop 20 is responsive to north-south tiltsand loop 50 is responsive to east-west tilts. The convergence isextremely rapid at the lower latitudes so that only two half-cycles ofswitching are required. The convergence is suflficiently rapid even atthe highest latitudes that gyro drift as a practical matter may becompletely eliminated in three or four half-cycles of switching back andforth.

In order to speed up the platform turn through 90 upon each opening orclosing of micro-switch 8, I impress the voltage across resistor 13 uponazimuth gyro 81 through a capacitor 14. Upon the closing of micro-switch8, the upper terminal of resistor 13 becomes positive relative to itslower terminal. This causes a predetermined charge to flow throughcapacitor 14 which rapidly slews the platform through approximately 90.When microswitch 8 opens, this same charge flows through capacitor 14 inthe opposite direction, rapidly turning the platform back 90approximately to its original orientation. The R-C time-constant ofcapacitor 14 and the resistance of the rapid slewing torquer of azimuthgyro 81 should be much smaller than the time interval between actuationsof micro-switch 8. The rapid slew circuit cannot affect the steady-stateoperation of azimuth gyro 81, since capacitor 14 blocks a steady flow ofdirect current.

The component of earth rate sensed by that gyro controllinggyro-compassing is w(cos L)(sin Z), where Z is the northerly azimuthalerror. The component of earth rate sensed by that gyro which does notcontrol gyrocompassing is w(cos L)(cos Z). It will be noted thatintroduced into the gyro which does not control gyrocompassing is thesignal 17 which is equal to w cos L. Accordingly, the drift errorintroduced into that gyro which does not control gyro compassing by anazimuthal error Z is w(cos L)(1-cos Z). For small angles Z, the quantity(1cos Z) may be approximately expressed as:

Also for small angles Z, sin Z is approximately equal to Z. If D is theinitial drift of gyro 21 and D is the residual second-orderuncompensated drift in gyro 51 and D is the residual fourth-order driftin gyro 21 after the energization of winding 9 and D is the residualeighthorder drift in gyro 51 after the subsequent de-energization ofwinding 9, and K represents w cos L, then:

and in general:

D =2K(D /2K) Accordingly, at the extremely high latitude of 88.1 with aninitial drift D in gyro 21 of 01 per hour where earth rate w is equal toper hour and cos L=cos 88.1=.033 then K=15(.033)-=0.5 and 2K=l. Hence,

This represents a ten-fold improvement over the probable initial driftin horizontal gyro 51. After switching to the lower position of thearmatures by the energization of winding 9,

D =1(0.1) =.000l per hour The drift in gyro 21 has been reduced by afactor of one thousand from its initial value. After returning to theupper positions of the armatures by tie-energizing Winding 9,

D =1(O.1) =.0000-0001 per hour The drift in gyro 51 has been reduced bya factor of ten million from its probable initial value. At lesserlatitu-des the convergence is so rapid that not more than one switchingoperation is required. After the gyros have been thus trimmed, thesystem may be reconnected in accordance with conventional stableplatform practice. However, all three velocity servos are disabled; andthe gyro drift cancelling signals appearing at the three potentiometerbrushes are retained.

It will be seen that I have accomplished the objects of my invention. Ihave provided a completely automatic system which generates trimmingsignals substantially eliminating gyroscope drift. My automaticgyroscope trimming system requires no auxiliary external equipment andpermits the trimming operation to be performed with the gyros mounted onthe platform. The gyros may be retrimmed without the necessity ofremoval from the platform. The trimming operation is accomplishedrapidly and expeditiously which permits of gyro retrimming each day oreven more frequently as may be required.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of myclaims. It is further obvious that various changes may be made indetails within the scope of my claims without departing from the spiritof my invention. It is, therefore, to be understood that my invention isnot to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. An inertial system including in combination a first and a second anda third gyroscope, a first and a second accelerometer, a first and asecond integrator, a platform, means mounting the gyroscopes andaccelerometers on the platform, the third gyroscope having a verticallydisposed input axis, the first and second gyroscopes having horizontallydisposed orthogonal input axes, the accelerometers having horizontallydisposed orthogonal sensitive axes, the input axis of the firstgyroscope being disposed orthogonally to the sensitive axis of the firstaccelerometer, means providing a signal, means coupling the firstaccelerometer and the first integrator to the first gyroscope, meanscoupling the second accelerometer and the second integrator to thesecond gyroscope, a two-state device, means responsive to a first stateof the device for coupling the first accelerometer to the thirdgyroscope and for aplying the signal to the second gyroscope and forcoupling the second accelerometer to the second integrator, and meansresponsive to the second state of the device for coupling the secondaccelerometer to the third gyroscope and for applying the signal to thefirst gyroscope and for coupling the first accelerometer to the firstintegrator.

2. An inertial system as in claim 1 including means responsive to achange in state of the device for applying to the third gyroscope amomentary signal having such time integral as to torque the platformrapidly through substantially 3. An inertial system as in claim 1including means for nulling the first integrator.

4. An inertial system as in claim 1 including means for causing thedevice to alternate periodically between states.

5. An inertial system including in combination a first and a second anda third gyroscope, a first and a second accelerometer, a platform, meansmounting the gyro scopes and accelerometers on the platform, the thirdgyroscope having a vertically disposed input axis, the first and secondgyroscopes having horizontally disposed orthogonal input axes, theaccelerometers having horizontally disposed orthogonal sensitive axes,the input axis of the first gyroscope being disposed orthogonally to thesensitive axis of the first accelerometer, a first and a secondintegrator, means providing a signal, means coupling the firstaccelerometer to the first integrator and to the first gyroscope, meanscoupling the first integrator to the first gyroscope, means coupling thesecond accelerometer to the second integrator and to the secondgyroscope, means coupling the second integrator to the second gyroscope,a two-state device, means responsive to a first state of the device forcoupling the first accelerometer to the third gyroscope and for applyingthe signal to the second gyroscope and for disabling tie firstintegrator, and means responsive to the second state of the device forcoup-ling the second accelerometer to the third gyroscope and forapplying the signal to the first gyroscope and for disabling the secondintegrator.

An inertial system as in claim 5 including means responsive to a changein state of the device for applying to the third gyroscope a momentarysignal having such time integral as to torque the platform rapidlythrough substantially 90.

7. An inertial system as in claim 5 including means for nulling thefirst integrator.

8. An inertial system as in claim 5 including means for causing thedevice to alternate periodically between states.

9. A three-axis inertial stabilizing system including in combination aplatform, first means comprising a first integrator for rotationallystabilizing the platform about a first axis, second means comprising asecond integrator for rotationally stabilizing the platform about asecond axis, third means for rotationally stabilizing the platform abouta third axis, the three axes being orthogonally disposed and the thirdaxis being vertically disposed, means providing a signal, a two-statedevice, means responsive to a first state of the device for coupling thefirst means to the third means and for applying the signal to the secondmeans and for disabling the first integrator, and means responsive tothe second state of the device for coupling the second means to thethird means and for applying the signal to the first means and fordisabling the second integrator.

16. An inertial system as in claim 9 including means responsive to achange in state of the device for rotating the platform rapidly throughsubstantially 90 about the third axis.

11. An inertial system as in claim 9 including for nulling the firstintegrator.

12. An inertial system as in claim 9 including means for causing thedevice to alternate periodically between states.

13. An inertial platform system including in combina tion a first and asecond and a third singledegree-of-freedom gyroscope each having aninput axis and a torquer and providing a gimbal-angle output, a firstand a second accelerometer each having a sensitive axis and providing anoutput, a first and a second and a third integrator each having an inputand providing an output, means providing a first signal equal to w sin Land a second signal equal to w cos L, where w is the constant rate ofrotation of the earth and where L is latitude, a platform having a datumplane, means mounting the gyroscopes and accelerometers on the platform,the input axes of the first and second gyroscopes and the sensitive axesof the accelerometers being disposed parallel to the datum plane, theinput axis of the third gyroscope being disposed orthogonally to thedatum plane, the input axis of the first gyroscope being orthogonallydisposed to the sensitive axis of the first accelerometer, the inputaxes IDEEU'IS of the first and second gyroscopes being orthogonallydisposed, the sensitive axes of the accelerometers being orthogonallydisposed, means responsive to the respective outputs of the first andsecond and third gyroscopes for controlling rotation of the platformabout the respective input axes of the first and second and thirdgyroscopes, a two-state device, means coupling the output of the firstaccelerometer and the output of the first integrator to the torquer ofthe first gyroscope, means coupling the output of the secondaccelerometer and the output of the second integrator to the torquer ofthe second gyroscope, means couplin the input and the output of thethird integrator and the first signal to the torquer of the thirdgyroscope, means responsive to a first state of the device for couplingthe output of the first accelerometer to the input of the thirdintegrator and for applying the second signal to the torquer of thesecond gyroscope and for coupling the output of the second accelerometerto the input of the second integrator, and means responsive to thesecond state of the device for coupling the output of the secondaccelerometer to the input of the third integrator and for applying thesecond signal to the torquer of the first gyroscope and for coupling theoutput of the first accelerometer to the input of the first integrator.

14. An inertial platform system as in claim 13 including cans responsiveto a change in state of the device for applying to the torquer of thethird gyroscope a momentary signal having such time integral as torotate the platform rapidily through substantially 90.

15. An inertial platform system as in claim 13 including means formilling the output of the first integrator.

1. An inertial platform system as in claim 13 including means forcausing the device to alternate periodically between states.

17. An inertial platform trimming system including in combination aplatform provided with two single-degreeof-freedom gyroscopcs havingorthogonally disposed input axes lying in a horizontal plane, atwo-state device, means responsive to a first state of the device forazimuthally orienting the platform such that the input axis of a firstgyroscope is aligned with north and for trimming the first gyroscope,and means responsive to second state of the device for azimuthallyorienting the platform such that the input axis of the second gyroscopeis aligned with north and for trimming the second gyroscope.

18. An inertial platform trimming system as in claim 17 including meansfor causing the device to alternate periodically between states.

19. An inertial platform trimming system including in combination aplatform having a datum plane and a reference line in the datum plane,means for rotationally stabilizing the platform such that the datumplane is horizontal, a twostate device, means for providing a first anda second trimming signal, means applying said signals to the stabilizingmeans, means responsive to a first state of the device for azimuthallyorienting the platform such that the reference line is aligned with acertain true direction and for varying the first signal and formaintaining the second signal constant, and means responsive to thesecond state of the device for changing the azimuthal orientation of theplatform by 90 and for varying the second signal and for maintaining thefirst signal constant.

20. An inertial platform trimming system as in claim 19 including meansfor causing the device to alternate periodically between states.

21. An inertial platform trimming system including in combination aplatform provided with a single-degreeof-freedom gyroscope having aninput axis disposed in a horizontal plane, a two-state device, means forproviding a trimming signal, means applying the signal to the gyroscope,means responsive to a first state of the device for azimuthallyorienting the platform such that the input axis is aligned with northand for varying the signal, and means responsive to the second state ofthe device for 9 changing the azimuthal orientation of the platform by90 and for maintaining the signal constant.

22. An inertial platform trimming system as in claim 21 including meansfor causing the device to alternate periodically between states.

References Cited UNITED STATES PATENTS 3,241,363 3/1966 Alderson et a1.

10 FOREIGN PATENTS 1,315,998 12/1962 France.

956,264 4/ 1964 Great Britain.

ROBERT B. HULL, Primary Examiner.

1. AN INERTIAL SYSTEM INCLUDING IN COMBINATION A FIRST AND A SECOND ANDA THIRD GYROSCOPE, A FIRST AND A SECOND ACCELEROMETER, A FIRST AND ASECOND INTEGRATOR, A PLATFORM, MEANS MOUNTING THE GYROSCOPES ANDACCELEROMETERS ON THE PLATFORM, THE THIRD GYROSCOPE HAVING A VERTICALLYDISPOSED INPUT AXIS, THE FIRST AND SECOND GYROSCOPES HAVING HORIZONTALLYDISPOSED ORTHOGONAL INPUT AXES, THE ACCELEROMETERS HAVING HORIZONTALLYDISPOSED ORTHOGONAL SENSITIVE AXES, THE INPUT AXIS OF THE FIRSTGYROSCOPE BEING DISPOSED ORTHOGONALLY TO THE SENSITIVE AXIS OF THE FIRSTACCELEROMETER, MEANS PROVIDING A SIGNAL, MEANS COUPLING THE FIRSTACCELEROMETER AND THE FIRST INTEGRATOR TO THE FIRST GYROSCOPE, MEANSCOUPLING THE SECOND ACCELEROMETER AND THE SECOND INTEGRATOR TO THESECOND GYROSCOPE, A TWO-STATE DEVICE, MEANS RESPONSIVE TO A FIRST STATEOF THE DEVICE FOR COUPLING THE FIRST ACCELEROMETER TO THE THIRDGYROSCOPE AND FOR APLYING THE SIGNAL TO THE SECOND GYROSCOPE AND FORCOUPLING THE SECOND ACCELEROMETER TO THE SECOND INTEGRATOR, AND MEANSRESPONSIVE TO THE SECOND STATE OF THE DEVICE FOR COUPLING THE SECONDACCELEROMETER TO THE THIRD GYROSCOPE AND FOR APPLYING THE SIGNAL TO THEFIRST GYROSCOPE AND FOR COUPLING THE FIRST ACCELEROMETER TO THE FIRSTINTEGRATOR.